Technological applications of superconductivity

Some of the technological applications of superconductivity include:

• the production of sensitive magnetometers based on SQUIDs
• fast digital circuits (including those based on Josephson junctions and rapid single flux quantum technology),
• powerful superconducting electromagnets used in maglev trains, Magnetic Resonance Imaging (MRI) and Nuclear magnetic resonance (NMR) machines, magnetic confinement fusion reactors (e.g. tokamaks), and the beam-steering and focusing magnets used in particle accelerators
• low-loss power cables
• RF and microwave filters (e.g., for mobile phone base stations, as well as military ultra-sensitive/selective receivers)
• fast fault current limiters
• high sensitivity particle detectors, including the transition edge sensor, the superconducting bolometer, the superconducting tunnel junction detector, the kinetic inductance detector, and the superconducting nanowire single-photon detector
• railgun and coilgun magnets
• electric motors and generators^[1]

Magnetic Resonance Imaging (MRI) and Nuclear Magnetic Resonance (NMR)

The biggest application for superconductivity is in producing the large volume, stable, and high magnetic fields required for MRI and NMR. This represents a multi-billion US$ market for companies such as Oxford Instruments and Siemens. The magnets typically use low temperature superconductors (LTS) because high-temperature superconductors are not yet cheap enough to cost-effectively deliver the high, stable and large volume fields required, notwithstanding the need to cool LTS instruments to liquid helium temperatures. Superconductors are also used in high field scientific magnets.

High-temperature superconductivity (HTS)

The commercial applications so far for high temperature superconductors (HTS) have been limited.

HTS can superconduct at temperatures above the boiling point of liquid nitrogen, which makes them cheaper to cool than low temperature superconductors (LTS). However, the problem with HTS technology is that the currently known high temperature superconductors are brittle ceramics which are expensive to manufacture and not easily formed into wires or other useful shapes.^[2] Therefore the applications for HTS have been where it has some other intrinsic advantage, e.g. in

• low thermal loss current leads for LTS devices (low thermal conductivity),
• RF and microwave filters (low resistance to RF), and
• increasingly in specialist scientific magnets, particularly where size and electricity consumption are critical (while HTS wire is much more expensive than LTS in these applications, this can be offset by the relative cost and convenience of cooling); the ability to ramp field is desired (the higher and wider range of HTS's operating temperature means faster changes in field can be managed); or cryogen free operation is desired (LTS generally requires liquid helium that is becoming more scarce and expensive).

HTS-based systems

HTS has application in scientific and industrial magnets, including use in NMR and MRI systems. Commercial systems are now available in each category.^[3]

Also one intrinsic attribute of HTS is that it can withstand much higher magnetic fields than LTS, so HTS at liquid helium temperatures are being explored for very high-field inserts inside LTS magnets.

Promising future industrial and commercial HTS applications include Induction heaters, transformers, fault current limiters, power storage, motors and generators, fusion reactors (see ITER) and magnetic levitation devices.

Early applications will be where the benefit of smaller size, lower weight or the ability to rapidly switch current (fault current limiters) outweighs the added cost. Longer-term as conductor price falls HTS systems should be competitive in a much wider range of applications on energy efficiency grounds alone. (For a relatively technical and US-centric view of state of play of HTS technology in power systems and the development status of Generation 2 conductor see Superconductivity for Electric Systems 2008 US DOE Annual Peer Review.)

Holbrook Superconductor Project

The Holbrook Superconductor Project is a project to design and build the world's first production superconducting transmission power cable. The cable was commissioned in late June 2008. The suburban Long Island electrical substation is fed by about 600-meter-long underground cable system consists of about 99 miles of high-temperature superconductor wire manufactured by American Superconductor, installed underground and chilled with liquid nitrogen greatly reducing the costly right-of-way required to deliver additional power.^[4]

Tres Amigas Project

American Superconductor was chosen for The Tres Amigas Project, the United States’ first renewable energy market hub.^[5] The Tres Amigas renewable energy market hub will be a multi-mile, triangular electricity pathway of superconductor electricity pipelines capable of transferring and balancing many gigawatts of power between three U.S. power grids (the Eastern Interconnection, the Western Interconnection and the Texas Interconnection). Unlike traditional powerlines, it will transfer power as DC instead of AC current. It will be located in Clovis, New Mexico.

Magnesium diboride

Magnesium diboride is a much cheaper superconductor than either BSCCO or YBCO in terms of cost per current-carrying capacity per length (cost/(kA*m)), in the same ballpark as LTS, and on this basis many manufactured wires are already cheaper than copper. Furthermore, MgB₂ superconducts at temperatures higher than LTS (its critical temperature is 39 K, compared with less than 10 K for NbTi and 18.3 K for Nb₃Sn), introducing the possibility of using it at 10-20 K in cryogen-free magnets or perhaps eventually in liquid hydrogen. However MgB₂ is limited in the magnetic field it can tolerate at these higher temperatures, so further research is required to demonstrate its competitiveness in higher field applications.

Notes